Nonlinear active noise control design has been the subject of constant research and development. A conventional noise attenuating method is to attenuate noises by actively combining, in amplitude, generated sounds against the noises. For example, noises can be cancelled by generating sounds equal in amplitude but opposite in phase to the noises through for example an FIR filter, radiating the sounds from a speaker, and combining the noises with the sounds opposite to the noises in amplitude.
Generally, it is known in the art of active noise control (ANC) systems, that such systems are used to electronically sense and cancel undesired noise (or vibration) from noise producing sources such as fans, blowers, electronic transformers, engines, etc. One methodology for sensing and cancellation involves a “collocated” approach where a sensor (such as a microphone) and an actuator (such as a speaker) are located along the same plane as the wave-front plane of the disturbance noise (or vibration).
Numerous approaches in the nonlinear active noise control design revolves around designing the controller directly using general nonlinear models like Volterra model without going through any nonlinear modeling process. However, the said approach does not reveal the degree of nonlinearity, a knowledge which is useful at the design stage. In addition, designing Volterra filters is computationally intensive.
CN101276207 patent titled “multivariable non-linear system prediction function control method based on hammerstein model” discloses a controlling method of a multivariate nonlinear system prediction function based on the hammerstein model, characterized in that the method includes the steps of establishing the hammerstein model according to the process characteristic and the input output data, solving the prediction function control rate of the multivariate linear subsystem according to the hammerstein model linear part model parameters, set values and practical process output of, solving the equation V(k)=F(U(k)) to obtain optimal control law U(k) according to the hammerstein model nonlinear part model parameters and the multivariate linear multivariate nonlinear system prediction function control rate, and solving and implementing the optimal control law according to multivariate nonlinear system prediction function controller. However, the controller is designed using state space approach and relies on the use of an optimization procedure over a prediction horizon. This optimization procedure is time consuming and limits the applicability in real time implementation. Moreover, the exact function used in the nonlinear part of the Hammerstein model is not specified.
GB2308898 patent titled “adaptive nonlinear controller for electromechanical or electroacoustic system” discloses a method that revolves around Volterra filters and includes an arrangement of the linear and nonlinear blocks to improve the use of processor memory and subsequently reduce the computational load. However, when compared to other controller structures, like the bilinear filters and functional link neural network, the controller design process using Volterra filters is computationally intensive due to the large number of parameters that needs to be identified.
U.S. Pat. No. 7,062,050 patent titled “preprocessing method for nonlinear acoustic system” discloses a method of processing an audio signal in a nonlinear acoustic system to reduce distortion in corresponding regenerated audio signals. Particularly, the present invention involves the design of a predistorter to compensate the effect of nonlinear distortion of the audio source which requires inverting the nonlinear model that causes the distortion. However, the method of modeling this nonlinear distorting function has not been clearly outlined. In addition, the type and degree of nonlinearity strength may have to be known in advance.
WIPO Patent Application WO/1997/050078 titled “nonlinear reduced-phase filters for active noise control” discloses the design of a non adaptive fixed controller using a nonlinear reset logic filter. In active noise control, adaptive filter is almost exclusively used due to the time varying nature of the noise.
There remains a need in the art for a method and apparatus to model and control nonlinearity of amplifier and loudspeaker.